Process for demetallization of carbon nanotubes

ABSTRACT

Processes are provided for removing metal-based catalyst residues from carbon nanotubes by contacting the carbon nanotubes with an active metal agent and carbon monoxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of application Ser. No. 11/946,044,filed on Nov. 28, 2007.

FIELD OF THE INVENTION

The present invention relates to processes for removing metalimpurities, such as metal-based catalyst residues, from carbonnanotubes.

TECHNICAL BACKGROUND

Carbon nanotubes (CNTs) are self-assembling nanostructures comprised ofgraphite sheets rolled up into cylinders. Such nanostructures are termedsingle-walled carbon nanotubes (SWNTs) if they are comprised of a singlecylindrical tube. CNTs comprising two or more concentric tubes aretermed double-walled carbon nanotubes (DWNTs) and multi-wall carbonnanotubes (MWNTs), respectively. The diameters of SWNTs typically rangefrom 0.4 nm to ca. 3 nm, and the lengths from ca. 10 nm to a fewcentimeters.

Whether made by laser ablation, chemical vapor deposition or othertechniques, the carbon nanotubes often contain catalyst residues fromtheir synthesis. In many applications, those catalyst residues aredetrimental to the final end-use application.

A variety of methods for de-metallization of carbon nanotubes have beendeveloped, but they typically rely upon the use of oxidizing conditionsin strong acids. Such conditions will remove much of the catalystresidue, and will even remove carbon species such as amorphous carbon.However, these aggressive processes can also result in severe damage to,and loss of, CNTs.

Chinese patent application CN1485271A discloses a method to remove Co,Ni and Fe in CNTs by heating them under hydrogen to 650° C. and then COgas at 150-200° C. before removing the metals under vacuum.

A non-destructive mild oxidation method of removing some impurities fromas-grown carbon nanotubes, including single-wall carbon nanotubes andmulti-wall carbon nanotubes, by H2O2 oxidation and HCl treatment hasbeen investigated (Feng et al., Chem. Phys. Lett. 2003, 375, 645-648).

An efficient, industrial scale purification process to remove theseimpurities is desired, as many of the applications of CNTs requirehighly-purified CNTs with low levels of damage to the CNT structure. Ahigh-yield method that removes the catalyst residues from the carbonnanotubes, that does not require strong acid oxidation, and that leavesthe carbon nanotubes in an unoxidized form, would be of considerablevalue.

SUMMARY OF THE INVENTION

One aspect of the present invention is a process comprising:

-   -   a) forming a first suspension of carbon nanotubes containing        metal residues in a first solvent;    -   b) contacting the first suspension with an active metal agent to        form a second suspension of carbon nanotubes containing metal        residues;    -   c) contacting the second suspension with carbon monoxide gas to        form carbon monoxide-treated carbon nanotubes; and    -   d) isolating the carbon monoxide-treated carbon nanotubes.

Another aspect of the present invention is a process comprising:

-   -   a) exposing carbon nanotubes comprising metal impurities to        flowing carbon monoxide gas;    -   b) raising the temperature of the carbon nanotubes and flowing        carbon monoxide from about 20° C. to a maximum temperature of        200° C. or lower, to produce volatile metal species and carbon        monoxide-treated nanotubes;    -   c) transporting the volatile metal species away from the carbon        monoxide-treated carbon nanotubes; and    -   d) isolating the carbon monoxide-treated carbon nanotubes.

DETAILED DESCRIPTION

The processes of the invention disclosed herein remove metal impurities,such as metal-based catalyst residues from carbon nanotubes (CNTs) undermild, reducing conditions. In some embodiments, the processes have theeffect of reducing the catalyst metal concentrations in the resultingCNT product, but do not substantially affect amorphous carbon or thenature of the carbon nanotubes present in the mixture.

One embodiment is a method for extracting contaminant metals from carbonnanotubes using an active metal to provide a reducing environment andcarbon monoxide to serve as an extracting ligand. The contaminant metalis removed from the carbon nanotubes as a soluble metal complex and thethus-formed de-metallated carbon nanotubes are then isolated.

In the processes disclosed herein, the carbon nanotubes containing themetal residues are exposed to an active metal agent prior to theexposure to CO. As used herein, an “active metal agent” includes metalswhose reduction reaction has a value 2.3 or more volts more negativethan that of the standard hydrogen electrode. Suitable metals includelithium, sodium, potassium, cesium, and magnesium. Organometalliccompounds, and in particular, metal alkyl compounds, are also suitableactive metal agents. Thus, lithium alkyls, magnesium alkyls and zincalkyls—for example, butyl lithium, dibutylmagnesium and diethylzinc—aresuitable active metal agents. Aluminum and boron alkyls are alsosuitable.

Exposure of the carbon nanotubes to the active metal agent typicallytakes place in a first suspension (a slurry) comprising the carbonnanotubes, a suitable solvent and optional other components. Exposure ofthe slurry to ultrasonic energy can be beneficial. The slurry can alsobe heated, with or without sonication. The reaction mixture formed bycontacting the carbon nanotube slurry with the active metal agent isreferred to herein as the “second suspension.”

In one embodiment, the slurry of carbon nanotubes is also sonicatedbefore the addition of the active metal agent.

In one embodiment, an electron-transporting species (e.g., naphthalene,anthracene or phenanthrene) is added to the slurry before sonication orheating.

When the second suspension is contacted with carbon monoxide, metalssuch as Ni, Co, Mo, Cr and Fe can readily be removed as their respectivecarbonyl complexes. Suitable temperatures typically include 0-150° C. Itis generally preferred to work at or above room temperature and theupper temperature limit is largely determined by the decompositiontemperature of the metal carbonyl(s) being formed. In one embodiment,the reaction is carried out between 20 and 100° C.

Exposure of the second suspension to carbon monoxide is convenientlycarried out at one atmosphere gas pressure (101 kilopascal), but thepressure can range from 10 to 10,000 kilopascal. Gaseous CO dissolves inthe solvent, where it serves to extract the metallic catalyst residues.The gas need not be pure carbon monoxide; for example, it can be dilutedwith nitrogen or other inert gas, or with hydrogen.

Exposure of metal-contaminated CNTs to CO can form volatile, and in somecases, toxic metal carbonyl species. Suitable precautions known to thoseskilled in the art can be taken to avoid exposure to Fe(CO)₅ or Ni(CO)₄.

The exposure of the carbon nanotubes to CO is carried out in asuspension comprising the CNTs and one or more liquid organic species,herein termed “solvents,” even though the carbon nanotubes do nottypically dissolve in these organic liquids. Suitable first and secondsolvents include aromatic solvents, i.e., hydrocarbon liquids thatcontain one or more benzene rings. Aromatic solvents can beunsubstituted or substituted with any functionality that does notinterfere with the metal removal process. Suitable functionalitiesinclude branched and linear alkyl and ether groups. Examples of suitablearomatic solvents include benzene, toluene, ethylbenzene, xylenes,mesitylene and mixtures thereof. Suitable first and second solvents alsoinclude non-aromatic solvents that are known to solubilize metalcations. Tetrahydrofuran and dioxane are particularly suitable.Functionalities such as hydroxyl, aldehyde, and carboxylate interferewith the process through reaction with the active metal and thus are notsuitable. Solvents such as ketones, aldehydes, alcohols, water, andorganic acids and their esters are known to react rapidly with activemetals and are thus not suitable for use in the slurry.

The suspensions can further comprise one or more “adjuvantligands.”Examples of adjuvant ligands include P- or N-containing ligandssuch as triphenylphosphine; 1,10-phenanthroline; 2,2′-bipyridyl;triphenylphosphite; and 1,2-bis(diphenylphosphine)ethane. Use of suchligands can facilitate the removal of metals by forming more stable COcomplexes.

After the metallic residues have been converted to soluble species byreaction with an active metal agent and CO, the de-metallated carbonnanotubes can be isolated by any of a variety of means. Filtrationthrough a fine-pored filter is particularly convenient, with the CNTsremaining on the filter. Alternatively, one can centrifuge the CNTsuspension and decant the supernatant solvent from the compacted carbonnanotubes.

Following the metal removal, one can re-suspend the CNTs in a solventthat is more compatible with the end-use application of the carbonnanotubes. For some applications, it is useful to handle the CNTs in arelatively volatile solvent or in a polar solvent that may not have beenappropriate for the initial CO-based extraction process. Thus it may beuseful to replace the extraction solvent with a new solvent by washingthe isolated CNT mass with the new solvent. For example, ethyl acetatecan be used to wash the isolated carbon nanotubes if the carbonnanotubes are going to be used in a photo-imageable paste formulation.Typically, the replacement of the extraction solvent is accomplishedwithout allowing the isolated mass of carbon nanotubes to dry.

In one embodiment, the isolated CNTs can be dried and optionallyannealed. “Annealing” refers to a process in which the CNTs are heatedunder an inert atmosphere to temperatures ranging from about 700 to1000° C. Alternatively, it may be useful to keep the CNTs wet or dampwith solvent to facilitate safe handling.

The processes disclosed herein can be used to reduce metal impurities inCNTs from 5-30 wt % to less than about 1 wt %, which is sufficient toallow the use of the CNTs in many advanced applications, such ascomposite materials, sensors, and nanoelectronic building blocks. Whiletypically applied to unpurified CNT materials, the processes can also beapplied to CNT materials that have been at least partially purified byanother method prior to being subjected to the processes herein.

The processes described herein possess several advantages over existingmethods. The processes yield CNTs with significantly reduced metalloadings and in high yield (80-90%). The reagents used typically havehigh selectivity toward the removal of metals and are alsonon-destructive to carbon nanotubes. By converting the metallicimpurities into soluble or volatile metal complexes, the impurities arereadily removed.

Further, the processes can be carried out as a solution-based process,involving mild conditions (e.g., 20-150° C.). It can be practiced as abatch, semibatch or continuous process (using continuous centrifugationto isolate the purified CNTs).

It has been found that passing CO gas over a heated sample of impure CNTmaterial can remove some metal impurities. In one embodiment of such aprocess, no solvent or active metal agent is required. In one embodimentof the process, the temperature for CO exposure is within the range of50 of 200° C. In another embodiment, the CNTs are exposed at elevatedtemperatures (>500° C.) to vapors of a lower-boiling active metal suchas Na, K, Rb, or Cs, then cooled to temperatures of 50 to 200° C., andthen exposed to CO. In one embodiment of the invention, the flow of COgas serves to transport the metal carbonyls out of the reaction area.

The CNTs produced can be used in a variety of applications, such as anelectrode of a fuel cell or battery, a heat sink or heat spreader, ametal-matrix composite or polymer-matrix composite in a printed circuitor as an electron emitter in a field emission display. Usefulformulations of the CNTs include: dispersions in a gas; dispersions in aliquid; dispersions in a solid; powders; pastes; and colloidalsuspensions.

The processes can be used to make a paste containing the purified CNTsthat is suitable for screen printing. “Screen printing” is a well-knownprinting technique. A printing screen is made of a piece of porous,finely woven stainless steel screen stretched over a frame. Areas of thescreen may be blocked off with a non-permeable material to form astencil, which is a positive of the image to be printed; that is, theopen spaces are where the paste or ink will appear. Alternatively, thescreen printing process may simple be used to print a uniform coatacross the entire surface of a substrate. The screen is placed atop thesubstrate such as glass or ITO. In a typical process, thescreen-printable paste is placed on top of the screen, and a fill bar isused to fill the mesh openings with ink. The fill bar begins at the rearof the screen and behind a reservoir of ink. The screen is lifted toprevent contact with the substrate and then using a slight amount ofdownward force the fill bar is pulled to the front of the screen. Thisfills the mesh openings with ink and moves the paste reservoir to thefront of the screen. A rubber blade moves the mesh down to the substrateas it is pushed to the rear of the screen. The paste that is in the meshopening is transferred by capillary action to the substrate in acontrolled and prescribed amount. One of more passes of the squeegee maybe required.

The paste used for screen printing typically contains carbon nanotubes,an organic medium, solvent, surfactant and either low softening pointglass frit, metallic powder or metallic paint or a mixture thereof. Therole of the medium and solvent is to suspend and disperse theparticulate constituents, i.e., the solids, in the paste with a properrheology for typical patterning processes such as screen printing. Thereare many such mediums known in the art. Examples of resins that can beused are cellulosic resins such as ethyl cellulose and alkyd resins ofvarious molecular weights. Butyl carbitol, butyl carbitol acetate,dibutyl carbitol, dibutyl phthalate and terpineol are examples of usefulsolvents. These and other solvents are formulated to obtain the desiredviscosity and volatility requirements. A surfactant can be used toimprove the dispersion of the particles. Organic acids such oleic andstearic acids and organic phosphates such as lecithin or Gafac®phosphates are typical surfactants.

A glass frit that softens sufficiently at the firing temperature toadhere to the substrate and to the carbon nanotubes is often employed. Alead or bismuth glass frit can be used as well as other glasses with lowsoftening points such as calcium or zinc borosilicates. Within thisclass of glasses, the specific composition is generally not critical. Ifa screen printable composition with higher electrical conductivity isdesired, the paste also contains a metal, for example, silver or gold.The paste typically contains about 40 wt % to about 80 wt % solids basedon the total weight of the paste. These solids comprise acicular carbonand glass frit and/or metallic components. Variations in the compositioncan be used to adjust the viscosity and the final thickness of theprinted material.

The emitter paste is typically prepared by three-roll milling a mixtureof carbon nanotubes, organic medium, surfactant, a solvent and lowsoftening point glass frit, metal oxide, metallic powder or metallicpaint or a mixture thereof.

The paste mixture can be screen printed by using a 165-400-meshstainless steel screen. The paste can be deposited as a continuous filmor in the form of a desired pattern. When the substrate is glass, thepaste is then fired in nitrogen or air. Higher firing temperatures canbe used with substrates which can endure them provided the atmosphere issubstantially free of oxygen. However, the organic constituents in thepaste are effectively volatilized at 350-450° C., leaving the layer ofcomposite comprised of carbon nanotubes and glass and/or metallicconductor. The carbon nanotubes appear to undergo no appreciableoxidation or other chemical or physical change during the firing innitrogen. Firing in air causes some oxidation, but oxidation is reducedwhen the concentration of metallic impurities is minimized.

In one embodiment, the polymer binder can be a “photopolymerizablepaste.” The photopolymerizable paste is printed as a uniform layeracross the substrate and then the image is generated by photoimagingprocesses. The non-exposed portions of the image can be removed bywashing, leaving the desired image on the substrate. A particularlyadvantageous pattern is an organized array of dots to be used as amultiplicity of field electron emitters for a display device. Ascreen-printed paste to be photopatterned contains a photoinitiator, adevelopable binder and a photohardenable monomer comprised, for example,of at least one addition polymerizable ethylenically unsaturatedcompound having at least one polymerizable ethylenic group. In oneembodiment, a paste composition for use as a screen printable pastecontains solids comprising carbon nanotubes, wherein the carbonnanotubes are less than 9 wt % of the total weight of solids in thepaste. In another embodiment, the carbon nanotubes comprise less than 5wt % of the total weight of solids in the paste. In a furtherembodiment, the carbon nanotubes comprise less than 1 wt % of the totalweight of solids in the paste. The pastes are useful in fabricatingelectron field emitters.

“Electron field emitters” or “electron field emission devices”containing the carbon nanotubes can be used in the cathodes ofelectronic devices such as triodes and in particular in field emissiondisplay devices. Such a display device comprises (a) a cathode using anelectron field emitter formulated with the de-metallated carbonnanotubes described herein, (b) a patterned optically transparentelectrically conductive film serving as an anode and spaced apart fromthe cathode, (c) a phosphor layer capable of emitting light uponbombardment by electrons emitted by the electron field emitter andpositioned adjacent to the anode and between the anode and the cathode,and (d) one or more gate electrodes disposed between the phosphor layerand the cathode. The use of an adhesive material to improve the emissionproperties of an electron field emitter is readily adapted to large sizeelectron field emitters that can be used in the cathodes of large sizedisplay panels. The electron field emitter may be a continuous area ormay be a plurality of electron field emitters printed as a series ofdots or other patterns. The electron field emitters may be addressedtogether to provide general lighting or may be addressed as individualemitters to provide information display.

The following examples are provided to demonstrate particularembodiments of the present invention. It should be appreciated by thoseof skill in the art that the methods disclosed in the examples whichfollow merely represent exemplary embodiments of the present invention.However, those of skill in the art should, in light of the presentdisclosure, appreciate that many changes can be made in the specificembodiments described and still obtain a like or similar result withoutdeparting from the spirit and scope of the present invention.

EXAMPLES

The carbon nanotubes used for these examples were either grown by thelaser-furnace ablation technique according to U.S. Pat. No. 6,183,714 B1or were purchased from Sigma Aldrich (St. Louis, Mo.). Metal residuelevels in the CNTs were determined by ashing at 600° C., dissolving theresidue in aqua regia (3:1 mix of HCl and HNO3 in water), and then usingInductively Coupled Plasma (ICP) elemental analysis. Unless otherwisestated, all operations other than final isolations were carried out inthe nitrogen atmosphere of a drybox (Vacuum Atmospheres, Hawthorne,Calif.) or using standard airless techniques. CNTs were isolated in astandard fume hood using techniques appropriate for the handling ofnanomaterials. All solvents, metals and organometallics were obtainedfrom Aldrich. Unless otherwise stated, percentages of materials areweight percent and temperatures are in degrees Celsius.

Example 1 Cobalt and Nickel Removal from CNTs

This example demonstrates the removal of nickel and cobalt catalystresidues from a sample of CNTs.

A sample of laser furnace CNTs (390 mg) containing about 5 wt % Co and 5wt % Ni catalyst residues was slurried in mesitylene (120 ml) and thensonicated at full power 50% duty cycle for 5 min. Then a sample ofpotassium metal (about 240 mg), cut into many small pieces with freshmetal surfaces exposed, was added to the mixture. Sonication was resumedat full power with a 50% duty cycle for 60 min. The CNT-K suspension inmesitylene was brought out of the drybox under nitrogen and exposed toair for several hours. It was then connected to a flow of CO (bubbledthrough), which egressed through an oil bubbler. The temperature wasincreased stepwise from 50 to 75 to 100° C. holding at each temperaturefor about 45 minutes. The time required to attain each set temperaturewas about 15 minutes. Upon cooling to room temperature, the CNTs wereisolated by filtration and washed with ethyl acetate (2×25 mL) beforesubmitting for ICP analysis of Co and Ni content. The sample wasanalyzed for 0.95 and 0.97 wt % Co and Ni, respectively.

Comparative Example A Cobalt and Nickel Removal from CNTs

This example demonstrates the attempted removal of nickel and cobaltcatalyst residues from a sample of CNTs without prior exposure to anactive metal agent.

A sample of laser furnace CNTs (390 mg) containing about 5 wt % Co and 5wt % Ni catalyst residues was slurried in mesitylene (120 ml), and thensonicated at full power 50% duty cycle for 5 min. The CNT suspension inmesitylene was brought out of the drybox under nitrogen and exposed toair for several hours. It was then connected to a flow of CO (bubbledthrough), which egressed through an oil bubbler. The temperature wasincreased stepwise from 50 to 75 to 100° C. holding at each temperaturefor about 45 minutes. The time required to attain each set temperaturewas about 15 minutes. Upon cooling to room temperature, the CNTs wereisolated by filtration and washed with ethyl acetate (2×25 mL) beforesubmitting for ICP analysis of Co and Ni content. The sample analyzedfor 3.10 and 3.08 wt % Co and Ni, respectively.

Example 2 Cobalt and Nickel Removal from CNTs

The procedure described for Example 1 was repeated using 141 mg of CNTand 0.061 g of potassium, except that there was no intervening exposureto air. The analytical results were 0.93 and 0.98 wt % Co and Ni,respectively. This result is very similar to the result with anintermediate exposure to air, showing that it had little effect.

Example Cobalt and Nickel Removal from CNTs

This example demonstrates the removal of nickel and cobalt catalystresidues from a sample of CNTs.

A sample of laser furnace CNTs (410 mg) containing about 3.90 wt % Coand 3.95 wt % Ni catalyst residues was slurried in mesitylene (120 ml)and then sonicated at full power 50% duty cycle for 5 min. Then a sampleof potassium metal (about 260 mg), cut into many small pieces with freshmetal surfaces exposed, was added to the mixture. Sonication was resumedat full power with a 50% duty cycle for 60 min. The CNT-K suspension inmesitylene was brought out of the drybox under nitrogen and CO wasbubbled through the suspension. The temperature was increased stepwisefrom 50 to 75 to 100° C. holding at each temperature for about 45minutes. The time required to attain each set temperature was about 15minutes. Upon cooling to room temperature, the CNTs were isolated byfiltration, giving a clear colorless filtrate. The CNTs were washed withethyl acetate (2×25 mL) before submitting for ICP analysis of Co and Nicontent, giving 0.47 wt % and 0.47 wt % respectively.

Example 4 Iron and Molybdenum Removal from CNTs

This example demonstrates the removal of iron and molybdenum catalystresidues from a sample of CNTs.

A sample of MWNTs (160 mg; OD=3-10 nm, ID=1-3 nm, length=0.1-10 μm,Aldrich 63, 654-1) containing about 1.09 wt % Fe and 0.22 wt % Mocatalyst residues was slurried in mesitylene (120 ml) and then sonicatedat full power 50% duty cycle for 5 min. Then a sample of potassium metal(about 43 mg), cut into many small pieces with fresh metal surfacesexposed, was added to the mixture. Sonication was resumed at full powerwith a 50% duty cycle for 60 min. The CNT-K suspension in mesitylene wasbrought out of the drybox under nitrogen, and CO was bubbled through thesuspension. The temperature was increased stepwise from 50 to 75 to 100°C. holding at each temperature for about an hour. The time required toattain each set temperature was about 15 minutes. Upon cooling to roomtemperature, the CNTs were isolated by filtration, giving a clear, greenfiltrate. The CNTs were washed with ethyl acetate (2×25 mL) beforesubmitting for ICP analysis of Fe and Mo content, giving 0.30 wt % and0.07 wt %, respectively.

Examples 5-14 Catalyst Removal from CNTs

The examples shown in Table 1 demonstrate that a variety of solvents,active metal agents and additives can be employed to remove catalystresidues from a variety of carbon nanotubes. Examples 5-12 used thelaser ablation carbon nanotubes. All of the experiments were carried outessentially as in Example 4. Example 13 used commercial multiwall carbonnanotubes (Aldrich 63, 654-1, OD=3-10 nm, ID=1-3 nm, length=0.1-10 μm).Example 14 used commercial multiwall carbon nanotubes (Aldrich 63,664-9), OD=20-50 nm, wall thickness=1-2 nm, length=0.5-2 μm).

TABLE 1 Examples 5-14 Active Starting Starting Ending Ending ExampleMetal Adjuvant wt % wt % wt % wt % No. Agent Solvent Ligand metal 1metal 2 metal 1 metal 2 5 Li THF — 3.96 Ni 3.90 Co 1.81 1.82 6 Kmesitylene — 3.96 Ni 3.90 Co 1.18 1.14 7 K mesitylene 1,10- 3.96 Ni 3.90Co 2.67 2.65 Phenanthroline 8 Li mesitylene Naphthalene 3.96 Ni 3.90 Co2.87 2.90 9 Cs mesitylene — 3.96 Ni 3.90 Co 0.72 0.69 10 Mg THF — 3.96Ni 3.90 Co 2.17 2.16 11 K Toluene — 3.96 Ni 3.90 Co 0.98 0.92 12 K1,4-dioxane 3.96 Ni 3.90 Co 0.79 0.74 13 K mesitylene 1.09 Fe 0.22 Mo0.03 0.07 14 K mesitylene 1.55 Ni — 0.84 —

Example 15 Cobalt and Nickel Removal from CNTs Using Butyl Lithium

This example demonstrates the removal of nickel and cobalt catalystresidues using butyl lithium as an active metal agent.

A sample of laser furnace CNTs (117 mg) containing about 3.90 wt % Coand 3.95 wt % Ni catalyst residues was slurried in mesitylene (120 ml)and then sonicated at full power 50% duty cycle for 5 min. Then a sampleof 1.6 M butyl lithium in hexanes (about 5 mL) was added drop-wise tothe mixture. Sonication was resumed at full power with a 50% duty cyclefor 60 min. The CNT-BuLi suspension in mesitylene was brought out of thedrybox under nitrogen and connected to a flow of CO (bubbled through),that egressed through an oil bubbler. The temperature was increasedstepwise from 50 to 75 to 100° C. holding at each temperature for aboutan hour. The time required to attain each set temperature was about 15minutes. Upon cooling to room temperature, the CNTs were isolated byfiltration giving a black, turbid filtrate. The CNTs were washed withethyl acetate (2×25 mL) before submitting for ICP analysis of Co and Nicontent, giving 0.97 wt % and 0.95 wt %, respectively.

Example 16 Cobalt and Nickel Removal from CNTs Using Dibutylmagnesium

Laser ablated CNTs (103 mg) and mesitylene (40 mL) were added to a glassjar in a drybox, resulting in a black suspension, which was sonicated atfull power and 50% duty cycle for 5 min. Di-n-butylmagnesium (2 mL) wasadded to the sonicated mixture. The mixture was further sonicated atfull power and 50% duty cycle for 90 min. The warm, black suspension wastransferred to a 100 mL, 3-neck, round-bottomed flask equipped with stirbar, glass thermo-well, gas adapter, and septum for injection. Thereaction flask was brought out of the drybox under nitrogen andconnected to a flow of CO (0.05 liters/min flow rate), which was bubbledthrough and allowed to egress through an oil bubbler. The temperaturewas increased stepwise from 50 to 75 to 100° C. holding at eachtemperature for about an hour. The time required to attain each settemperature was about 15 minutes. The CNTs were isolated via filtrationand washed with ethyl acetate (2×25 mL), transferred to a vial, andsubmitted for ICP analysis. The starting metal concentrations were:cobalt: 3.90 wt % and nickel: 3.96 wt %. The final metal concentrationswere: cobalt: 0.66 wt % and nickel: 0.65 wt %.

Example 17 Cobalt and Nickel Removal from CNTs Using Diethylzinc

Laser ablated CNTs (105 mg) and mesitylene (40 mL) were added to a glassjar in a drybox, resulting in a black suspension, which was sonicated atfull power and 50% duty cycle for 5 min. Diethylzinc (2 mL) was added tothe sonicated mixture. The mixture was further sonicated at full powerand 50% duty cycle for 90 min. The warm, black suspension wastransferred to a 100 mL, 3-neck, round-bottomed flask equipped with stirbar, glass thermo-well, gas adapter, and septum for injection. Thereaction flask was brought out of the drybox under nitrogen andconnected to a flow of CO (0.05 liters/min flow rate), which was bubbledthrough and allowed to egress through an oil bubbler. The temperaturewas increased stepwise from 50 to 75 to 100° C. holding at eachtemperature for about an hour. The time required to attain each settemperature was about 15 minutes. The CNTs were isolated via filtrationand washed with ethyl acetate (2×25 mL), transferred to a vial, andsubmitted for ICP analysis. The starting metal concentrations were:cobalt: 3.90 wt % and nickel: 3.96 wt %. The final metal concentrationswere: cobalt: 1.35 wt % and nickel: 1.41 wt %.

Example 18 Formulation of CNT Paste and Preparation of a Field EmissionDisplay

Laser-ablation grown single wall carbon nanotubes were purified asdescribed in Example 1. The carbon nanotube powder was sonicated in amixture of ethyl acetate and terpineol to create a slurry which was thenincorporated into photoimageable paste components via roll-milling.Fodel® photoimageable paste is available from E. I. du Pont de Nemoursand Company, Wilmington, Del. It contains a photoinitiator andphotomonomers. The resulting paste was then screen printed ontopatterned ITO-coated glass substrates, 2″×2″ in size. The paste wasimaged pattern-wise with exposure to UV light (100 mJ/cm2) and developedin a 4:1 NMP:H2O solution for ˜1 min. and then rinsed with deionizedwater to reveal the dots patterned onto the substrate. The samples werethen fired in a belt furnace in either N2 at 420° C. or air at 400° C.The peak time at temperature in either case was on the order of 17minutes. After firing, the carbon nanotube Fodel® composition forms anadherent coating on the substrate.

To test for emission, a layer of material was removed from the surfaceof the fired substrates using a tape activation process (US 200610049741A1) in which a piece of Scotch® Magic® Tape, (#810, 3M Company) wasapplied to and contacted with the electron field emitter and thenremoved. The substrate was then placed into a diode configuration withan ITO-coated phosphor plate at a separation of 620 μm. The diode wasplaced into a vacuum chamber and electrically connected to a voltagesource and an ammeter. The chamber was pumped to ˜3.5×10-6 Torr. Anautomated system was used to increase the voltage applied to the diode(from 0 to up to 3200V) while measuring the resulting emission current.The field required to obtain 36 μA was under 2 V/μm for the sample firedin nitrogen and 3.6 V/μm for the sample fired in air. Samples withhigher loadings of metal generally emit under similar conditions whenfired in nitrogen, but often will not emit at all when fired in air.

Example 19 Vapor-Transport of Volatile Metal Species Away from CNTs

A sample of laser grown carbon nanotubes (about 0.1 g) was placed in analumina boat (Coors, Boulder, Colo.) and the boat was inserted into aquartz tube. The quartz tube was placed into a tube furnace (Lindberg,Watertown, Wis.). The quartz tube was then flushed with carbon monoxide.With a constant CO flush, the temperature of the furnace was slowlyramped over a period of two hr to 200° C., and then held at thattemperature for another hour. The sample was then allowed to cool slowlywith continued CO flush. Upon opening the furnace, it was observed thatsome of the metal formerly on the CNTs was now on the walls of thequartz tube as a fine mirror downstream from the CNTs. Volatile metalspecies had been transported away from the CNTs.

1. A process comprising: a) exposing carbon nanotubes comprising metal impurities to flowing carbon monoxide gas; b) raising the temperature of the carbon nanotubes and flowing carbon monoxide from about 20° C. to a maximum temperature of 200° C. or lower, to produce volatile metal species and carbon monoxide-treated nanotubes; c) transporting the volatile metal species away from the carbon monoxide-treated carbon nanotubes; and d) isolating the carbon monoxide-treated carbon nanotubes.
 2. The process of claim 1 wherein prior to exposing the carbon nanotubes to flowing carbon monoxide gas, the carbon nanotubes are first exposed to an active metal agent selected from the group consisting of lithium, sodium, potassium, cesium, and combinations thereof.
 3. The process of claim 1, wherein the process further comprises annealing the carbon monoxide-treated carbon nanotubes.
 4. The process of claim 1, wherein the carbon nanotubes are made by laser ablation.
 5. The process of claim 1, wherein the carbon nanotubes are made by chemical vapor deposition.
 6. A composition comprising the isolated carbon nanotubes made by the process of claim
 1. 7. A screen printable paste comprising the composition of claim 6, a solvent, and a binder.
 8. A photopolymerizable paste comprising the composition of claim 6, a solvent, a binder, a photoinitiator, a developable binder and a photohardenable monomer.
 9. An electron field emission device comprising the composition of claim
 6. 10. A plurality of field emitters comprising the composition of claim
 6. 11. The process of claim 1 wherein the metal residues comprise catalyst metal residues. 